1. Field of the Invention
This invention relates to an apparatus for, and method of, determining an angle between an element and a magnetic field. In particular, the invention relates to the measurement, with a high degree of accuracy, of the angle between the rotational axis of a rotating sample holder, and the direction of the magnetic field in nuclear magnetic resonance spectrometers, most particularly when the nuclear magnetic resonance device is used for analysis of solid-state samples.
2. Description of the Prior Art
In solid-state nuclear magnetic resonance spectroscopy (NMR) experiments the sample and sample holder (such holder is also called a rotor) are often rotated, around the principal axis of the rotor, at a precise angle (usually tan−1√2=54.74° also known to those skilled in the art of NMR spectroscopy as the “Magic Angle”) with respect to the direction of the main magnetic field. This technique can be used to increase the resolution achieved in a NMR experiment, improving the quality and analysis of the spectrum generated. The quality of data generated in such NMR experiments is highly sensitive to deviations from the desired angle. Routine experiments are sensitive to errors of around ±0.1°, and some NMR experiments are extremely sensitive to a precise setting of this angle with deviations of ˜±0.002° adversely affecting the quality of spectrum generated.
When conducting an NMR experiment the rotor is seated in the stator of a probe which is inserted into the bore of the NMR magnet. The rotor and stator are engineered to provide a precise fit of the rotor into the stator with a tolerance of a few micrometers. The rotor axis and stator axis are thus within millidegrees of absolute alignment with each other. Usually the angle at which the stator, and hence rotor, is held with respect to the probe is adjusted mechanically by turning a knob on the base of the NMR probe. Other adjustment systems may also be used. The angle set between the stator axis and magnetic field direction is estimated based on the orientation of the probe with respect to the magnetic field direction.
None of the currently available systems for adjusting rotor angle provide direct or sufficiently accurate feedback on the actual angle between the magnetic field direction and the stator axis or rotor axis. The only means of directly and accurately measuring the angle between the rotor axis and the magnetic field direction is to undertake an NMR experiment followed by careful analysis of the results to assess whether actual angle was, in fact, at the intended angle setting. This analysis is a straightforward task for suitable operators as, for example, the shape of the spectral lines generated is highly sensitive to the spinning angle.
For these reasons, whilst the operator will seek to position the rotor at the desired (specific) angle, there is considerable difficulty in setting the angle between the rotational axis and the direction of the magnetic field precisely within the desired error limits of ˜±0.1° for many NMR experiments, up to ˜±0.002° for a number of particularly sensitive experiments. In practice setting the angle to the desired value for a particular experiment is a trial and error process. Furthermore, small angular deviations from the desired spinning angle that may occur due to mechanical slip or thermal expansion after the angle has been set by the operator cannot be detected, using currently known systems and methods, until the NMR experiment has been completed and analysed.
This current limitation of NMR machines, with respect to Solid State NMR (SS NMR) means that a repeated series of sequential experiments, analysis and mechanical adjustment of the probe angle may be necessary to achieve the desired angle and hence resolution. Thus the current inability to accurately measure and therefore adjust rotor angle prior to commencing an NMR experiment is detrimental in terms of both operator and spectrophotometer time. Current technology also limits usage of angle-sensitive NMR techniques to highly skilled operators. Additionally, for short-lived samples, there may be insufficient time to undertake such a series of adjustments and the opportunity to collect data of a precious sample may be lost. These factors limit the ability of such techniques to be automated and standardized, and inhibit uptake of these techniques by broader scientific and industrial user groups.
Therefore there is a clear need in the field of solid state NMR spectroscopy for a means of accurately determining the angle between the rotational axis of the rotor (sample holder) and the direction of the magnetic field, preferably in real time.
There is also a clear need in the field of solid state NMR spectroscopy for a means of accurately adjusting the angle between the rotational axis of the rotor (sample holder) and the direction of the magnetic field automatically.